A refrigerant piping unit used for providing a refrigerant passage of a refrigerant cycle apparatus, is described in U.S. Pat. No. 6,330,739, for example. The refrigerant piping unit has a pipe defining a refrigerant passage therein and a joint for coupling the pipe. The pipe is bent substantially perpendicular to include a bent portion with a small radius of curvature so as to make the pipe compact. An end of the pipe is inserted in the joint member. The pipe has a fixing flange adjacent to the bent portion. The pipe is fixed to the joint member through the fixing flange. Further, the end of the pipe is formed with a groove and an O-ring is fitted in the groove.
In the described piping unit, an inner surface of the end of the pipe is uneven since the fixing flange and the groove are formed. Such an uneven inner surface causes disturbance of refrigerant, resulting an increase in pressure loss and a decrease in cooling performance of a system to which the refrigerant piping unit is employed.